Precipitation Preparation of High Surface Area and Porous Nanosized

Article pubs.acs.org/IECR

Precipitation Preparation of High Surface Area and Porous Nanosized ZnO by Continuous Gas-Based Impinging Streams in Unconfined Space Caijin Zhou,†,‡ Yujun Wang,*,† Le Du,*,‡ Hongbao Yao,† Jingchuo Wang,† and Guangsheng Luo† †

The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China The State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Membrane Science and Technology, Beijing University of Chemical Technology, Beijing 100029, China

‡

S Supporting Information *

ABSTRACT: Using the precipitation method, we propose a new process for the preparation of high specific surface area and large pore volume ZnO nanoparticles in unconfined space with NH4HCO3 and ZnSO4·7H2O as the reactants. The mixing performance of the reaction system was improved by gas atomization and continuous gas-based impinging streams before the precipitation reaction. By virtue of the gas environment and gas division, the obtained nanoparticles have a very good dispersion performance. Under optimal conditions, the ZnO nanoparticles were synthesized with a surface area of 88.89 m2/g, an average diameter of 7 nm, and a pore volume of 0.68 cm3/g. The influences of ZnSO4 concentration, pressure, and gas−liquid ratio on the properties of the synthesized nanoparticles were studied. This study aims to provide a feasible and economical way to produce better properties in nanosized ZnO particles, which are widely applied as a new, multifunctional material.

1. INTRODUCTION In recent years, ZnO nanoparticles have attracted much attention as a new, multifunctional material.1,2 Because of its many novel characteristics, nanosized ZnO is more widely applied compared with its bulk counterparts, in fields such as catalysis, biomedicine, and photoelectronics.3−7 In particular, ZnO nanoparticles with high specific surface area and porosity are an important catalytic activity component for the synthesis of methanol.8,9 Currently, the precipitation method is the main method for the synthesis ZnO nanoparticles in industry due to its low cost and simple operation. Nanoparticles with high specific surface area and large pore volume are more highly favored.10 However, ZnO nanoparticles tend to agglomerate with the direct precipitation method, which greatly affects the quality of the product. Therefore, it is very important to develop a novel process for the preparation of nanoscaled ZnO particles with high specific surface area and porosity. When the precipitation reaction is used to synthesize ZnO nanoparticles, mixing performance and a homogeneous supersaturation environment are very important in the nucleation and growth process of the nanoparticles.11 Based on the theory of nucleation-growth, a high degree of saturation is beneficial to rapid nucleation and reduced growth, thereby producing nanoparticles with a smaller diameter and a smaller particle size distribution.12−14 As the diameter of the nanoparticles decreases, the particles have higher surface energy, which makes the particles more prone to agglomerate during the process of © 2016 American Chemical Society

synthesis. The high degree of aggregation results in a decrease of surface area and large particles. However, mixing the reactants homogeneously before the precipitation reaction can greatly reduce the degree of aggregation caused by the increased saturation.15,16 There are some researchers who have proposed a method of microscaled dispersion to demonstrate that this approach can significantly reduce particle aggregation in the process of synthesizing by the precipitation method. With the basic law of microscale mixing, Du et al. proposed a new process for controlled production of 40 nm nanoparticles by injecting micro air bubbles into the reactants.17 With the addition of gas, the mixing effect of the reaction before the precipitation reaction was obviously enhanced. Using a microreactor, Wang et al. produced a high specific surface area with a throughput of 0.997 g/min.18 Dispersion of the membrane also has a good mixing effect on the reaction solution. In addition, Huang et al. added CO2 to the reactants in the microreactor to improve further the mixing performance and obtain nanoparticles with better properties.19 Microscaled dispersion has very good application prospects. However, it has the limitations of having a small amount throughput and being easily blocked. Therefore, we transferred the generation of position of the particle to unconfined space Received: Revised: Accepted: Published: 11943

August 31, 2016 October 24, 2016 October 27, 2016 October 27, 2016 DOI: 10.1021/acs.iecr.6b03348 Ind. Eng. Chem. Res. 2016, 55, 11943−11949

Article

Industrial & Engineering Chemistry Research

Figure 1. Experimental setup to obtain precursor particles: (1) compressed air bottle; (2) nanosized pressure nozzle; (3) receiving device; (4) mechanical agitator.

Beijing Chemicals Company (Beijing, P. R. China). Both reactants were of analytical grade. The major component of the experimental setup (Figure 1) was a nanosized spray device comprising two nozzles with a nozzle diameter of 1 mm. A rotor flow meter was used to measure the liquid flow rate and gas flow rate. A stable source of gas was provided by air cylinders. A precursor particle receiving device was constructed from a long-stem funnel and a three-necked boiling flask. 2.2. Preparation of ZnO Nanoparticles. Before the experiment, the reactants were dissolved in deionized water. The pressure in the atomization device and the atmospheric pressure were disparate, which was employed as the driving force to bring the two reactants into the spray device. Then, compressed air with a pressure of 0.4 MPa individually atomized the two reactants in the nozzle. The atomized reactants in the form of small droplets were carried out of the nozzle by the fast air and collided in unconfined space (Figure 1). When the two air streams carrying small droplets collided, precipitation reactions occurred and the precursors were synthesized. The fast air streams acting as carrier gases brought the precursors into a three-necked boiling flask. In the threenecked boiling flask, the precursor solution was stirred for 1 h. The precursors were washed 4 times with deionized water and dried at 100 °C for 2 h. In the end, the dried precursors were calcined at 400 °C for 2 h and ZnO nanoparticles were obtained. Figure 2 shows the procedure for synthesizing ZnO nanoparticles that was used in this study. 2.3. Characterization. The structural properties and crystal size of the synthesized ZnO nanoparticles were analyzed using X-ray diffraction analysis (XRD, Rigaku Corporation D/max-R B) with a voltage of 40 kV. Transmission electron microscopy

and used the effect of higher pressure and faster air atomization by carrier gas flow of high-speed transmission while ensuring an efficient and similar mixing effect to guarantee the performance of the generated nanoparticles and increase the throughput. Moreover, the gas divided the reactants into droplets and the effect of the gas environment reduced the interaction between particles to obtain the production with low agglomeration. Therefore, we propose to take the gas as the continuous phase atomization reaction solution in which reactants form very small droplets and carried by the gas into the unconfined space. Then the two air flows collided and the precipitation reaction occurred in the unconfined space between small droplets for synthesizing ZnO nanoparticles. In this study, a new process for the preparation of zinc oxide nanoparticles was developed using continuous gas phase impingement and precipitation. Gas division and impinging streams have greatly improved the mixing intensity of reaction systems. Furthermore, mixing uniformity and the addition of gas can effectively reduce the aggregation of nanoparticles in the precipitation process. The effect of experimental variables on the properties of particles is studied in the experiments. By this new synthesis process, we have successfully prepared nanosized ZnO particles with a high specific surface area and porosity. This work aims to provide a straightforward and economical way for producing better properties of ZnO nanoparticles in industry.

2. EXPERIMENTAL SECTION 2.1. Materials and Apparatus. The reactants used in the experiments, zinc sulfate heptahydrate (ZnSO4·7H2O) and ammonium bicarbonate (NH4HCO3), were purchased from 11944

DOI: 10.1021/acs.iecr.6b03348 Ind. Eng. Chem. Res. 2016, 55, 11943−11949

Article

Industrial & Engineering Chemistry Research

and sonicated in an ultrasonic water bath for 10 min. Then the suspension was added into a 30 mL of MO solution (20.59 ppm), and the mixture was magnetically stirred in the dark for 30 min to achieve an adsorption−desorption equilibrium of MO on the surfaces of the ZnO nanoparticles. After equilibrium was reached, the mixture was irradiated under UV light. At different irradiation intervals, the mixture was sampled and centrifuged to remove ZnO nanoparticles. The concentration of MO was determined at a wavelength of 450 nm with a UV−vis spectrophotometer. The photocatalytic degradation efficiency was calculated by using the following equation:

Figure 2. Procedure for synthesizing ZnO nanoparticles.

R (%) = [MO]0 − [MO]/[MO]0 × 100

(TEM) analysis for production was performed using a JEOL JEM-200CX microscope. The Brunauer−Emmet−Teller (BET, Quantachrome autosorb-1) was used to obtain the specific surface area and pore volume of the nanosized ZnO particles. The SEM images of nanoparticles were observed using scanning electron microscopy (SEM, JEM-630F, Japan). 2.4. Photocatalytic Reaction. Photocatalytic efficiency of the obtained ZnO nanoparticles was measured by decomposing methyl orange (MO) under UV irradiation. The synthesized ZnO particles (90 mg) were dispersed in distilled water (5 mL)

(1)

The experiments were conducted at room temperature, and the pH value of solution was neutral.

3. RESULTS AND DISCUSSION 3.1. Effect of ZnSO4 Concentrations on the Diameters of Nanoparticles. Figure 3 shows the TEM images and corresponding particle size distribution of zinc oxide nanoparticles synthesized at different concentrations. Mixing

Figure 3. TEM image and corresponding particle size distribution of ZnO nanoparticles. The a, c, and e TEM images correspond to the 0.3, 0.5, and 0.6 mol/L concentrations in Table 1. 11945

DOI: 10.1021/acs.iecr.6b03348 Ind. Eng. Chem. Res. 2016, 55, 11943−11949

Article

Industrial & Engineering Chemistry Research

The enormous variation in particle diameter and pore volume is mainly caused by the increase of the gas sheer force on the reactants. Figure 5 demonstrates the relationship

performance and suitable concentration have a great influence on the particle size distribution and dispersion of nanoparticles. At a pressure of 0.4 MPa and ZnSO4 concentration of 0.5 mol/ L, the desired properties of nanoparticles with uniform particle size distribution and better dispersion were obtained. The influence of ZnSO4 concentration on average particle diameter and specific surface area is illustrated in Table 1. As Table 1. Surface Areas and Diameters of ZnO Nanoparticles at Different ZnSO4 Concentrations CZnSO4 (mol/L)

CNH4HCO3 (mol/L)

V (mL/min)

P (MPa)

surface areas (m2/g)

0.3 0.4 0.5 0.6 0.8

0.6 0.8 1.0 1.2 1.6

20 20 20 20 20

0.4 0.4 0.4 0.4 0.4

66.54 65.98 83.78 52.76 46.28

average particles diameter (nm) 11 9 8 14 17

Figure 5. Schematic illustration of the gas atomization effect on the reactant.

shown in Table 1, the crystal size is small with a lower ZnSO4 concentrations. Under the low ZnSO4 concentrations, there was high saturation and a tendency for crystal nucleation. However, when the concentration was high ([ZnSO4] ≥ 0.5 mol/L), the nanoparticle diameters increased. It is may due to that, when the degree of saturation exceeds a certain value, the mixing intensity is not sufficient, resulting in a heterogeneous reaction. In the process of synthesis, high concentrations promote the growth of particles and agglomeration, whereas low concentrations promote the nucleation of nanoparticles. As a result, the uniformity of the diameters of the synthesized nanoparticles and good dispersion cannot be guaranteed, which in turn results in particles with large average diameters and surface areas. In the following experiment, the ZnSO 4 concentration is 0.8 mol/L for better observing the effect of operating conditions on the properties of particles. 3.2. Effect of Pressure on the Process. In Figure 4, the effect of pressure on the average particle diameter and pore volume is shown. With a change in pressure ranging from 0.3 to 0.5 MPa, the average diameter decreased, but the pore volume of the nanoparticles increased. The variation is particularly significant when the pressure is increased to 0.5 MPa.

between pressure and gas division. The schematic illustration shows the different gas atomization effects on the reactants between a pressure of 0.4 and 0.5 MPa. With an increase in pressure, the sheer force of the gas on the reaction solution will be more potent. It results in a better gas atomization effect and generates more small droplets with the lower and appropriate concentrations of reactants. Therefore, when a droplet impinged and caused precipitation in the unconfined space, the reaction solution had already been mixed well. It is better to obtain nanoparticles with smaller particle diameters. Gas division and the existence of the gas environment also has an impact on the dispersion of particles. As shown in Figure 6, a pressure increase leads to a decrease in the degree of particle aggregation. It appears that the effect of gas division and gas environment effectively reduces the interaction between particles during the process of synthesis. Figure 7 shows the TEM images of the ZnO nanoparticles produced at a pressure of 0.4 and 0.5 MPa. With an increase in pressure, the effect of gas division on the reactants makes the product more likely to have a net-like structure. The particles are connected in a net formation that greatly reduces particle aggregation; therefore, the synthesized particles have a high specific surface area and porosity. Compared with other methods, the production of ZnO nanoparticles in unconfined space yields particles with excellent qualities. For example, by using urea as a precipitation agent for the synthesis of nanoparticles with high specific surface area, Boz et al. obtained zinc oxide particles with a surface area of 67.5 m2/g.10 With ethanol as a solvent, Liu et al. carried out self-assembly synthesis to obtain zinc oxide particles with a pore volume of 0.19 cm3/g.20 3.3. Effect of Gas Liquid Ratio on the Average Diameter and Pore Volume. The effect of the ratio of gas volume flow to liquid volume flow on the average particle diameter and the pore volume are shown in Figure 8. Under the same gas volume flow, as the liquid volume flow increased, the gas liquid ratio and the pore volume of the product decreased, but the particle size grew larger. The reason for this phenomenon may be that, under the same pressure, the handling capacity of the spray device increased as the liquid volume flow increased, resulting in a

Figure 4. Effect of pressure on the diameter of ZnO nanoparticles and pore volume. 11946

DOI: 10.1021/acs.iecr.6b03348 Ind. Eng. Chem. Res. 2016, 55, 11943−11949

Article

Industrial & Engineering Chemistry Research

Figure 6. SEM images of ZnO nanoparticles synthesized at various pressures: (a) 0.3 MPa; (b) 0.4 MPa; (c) 0.45 MPa; (d) 0.5 MPa. Other conditions: [ZnSO4] = 0.8 mol/L, [NH4HCO3] = 1.6 mol/L, V = 20 mL/min.

Figure 7. (E and f) TEM images of the ZnO nanoparticles correspond to the panel b and d SEM images in Figure 6.

decrease in the atomization performance. This could lead to an increase in the degree of saturation in the unit reaction space, but in this case, the mixing intensity is insufficient. Ferreira et al. found that a change in the gas−liquid ratio, caused a change in the mixing intensity of the reactant, by changing the flow pattern of the fluid in the atomization chamber.21,22 If the gas− liquid ratio is high, then the atomizer mixing performance of the reactants is poor. At a high degree of saturation, mixing intensity is insufficient, and the particles tend to aggregate. As shown in Figure 9, with an increase of liquid volume flow from 15 to 50 mL/min, the dispersion performance of synthesized nanoparticles grew worse. Thus, the nanoparticles have better qualities when they are synthesized at a lower liquid volume flow. 3.4. Comparison with other Mixing Methods. The preparation method used in this study was compared with other methods for preparing ZnO nanoparticles. This comparison is exhibited in Table 2. Compared with other methods, the present method can be carried out in unconfined space, which results in nanoparticles with better qualities. The diameters of particles synthesized with the present method are smaller than

Figure 8. Effect of gas liquid ratio on the diameter and pore volume of ZnO nanoparticles. Other operation conditions: [ZnSO4] = 0.8 mol/ L, [NH4HCO3] = 1.6 mol/L, P = 0.4 MPa.

11947

DOI: 10.1021/acs.iecr.6b03348 Ind. Eng. Chem. Res. 2016, 55, 11943−11949

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Industrial & Engineering Chemistry Research

Figure 9. SEM images of the ZnO nanoparticles. Images a−d correspond to liquid volume flow of 20, 30, 40, and 50 mL/min.

Table 2. Comparison with Different Mixing Methods for Synthesizing ZnO Nanoparticles mixing method impinging stream, bubbles in microreactor19 micromixer23 microreactor18 this experiment

reaction temperature

average diameter (nm)

surface area (m2/g)

reaction space

room temperature

11

82.38

confined

heating room temperature room temperature

18−436 9.6

65.89

confined confined

7

88.89

unconfined space

those prepared by other methods. In the process of synthesizing the precursor, the precipitation reaction occurred in unconfined space, which avoids the problem of blocking in the pipeline. It is easy to apply industrial production in unconfined space. This demonstrates that the present method is better in both mixing performance and particle qualities. 3.5. Application of Obtained ZnO in Photocatalytic Degradation. The photocatalytic degradation efficiency of methyl orange is evaluated by calculating the R value as explained in section 2.4. Figure 10 shows the resulting curves of the photocatalytic degradation rates versus irradiation times using ZnO with different specific surface areas. Obviously, the degradation rates increase with an increasing specific surface area of ZnO under the same reaction time. Specifically, as the specific surface area increase from 52.24 to 83.39 m2/g, the degradation rates increase from 34.5% to 98.8%. This phenomena could be explained that higher surface areas tended to bring about more active sites on the surfaces of ZnO nanoparticles, thus resulting in a higher photocatalytic activity.

Figure 10. Resulting curves of the photocatalytic degradation rates versus irradiation times using ZnO with different specific surface areas.

In addition, the quantity of MO absorbed onto the surface of ZnO nanoparticles also increased. Similar phenomena and corresponding explanation were also reported in the work of Kim.24 Furthermore, the R value of as-prepared ZnO with the highest surface area reached 91.6% after 60 min of UV light irradiation. In particular, it should be noted that the R value reported was just 70% in the work of Huang et al.19 and 86.6% in the work of Sanna et al.25 under the same experimental conditions, indicating that the ZnO nanoparticles prepared in this study exhibit a superior photocatalytic activity. 11948

DOI: 10.1021/acs.iecr.6b03348 Ind. Eng. Chem. Res. 2016, 55, 11943−11949

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Industrial & Engineering Chemistry Research

(6) Wang, Z. L. Nanostructures of zinc oxide. Mater. Today 2004, 7, 26−33. (7) Khan, Y.; Durrani, S. K.; Mehmood, M.; Ahmad, J.; Khan, M. R.; Firdous, S. Low temperature synthesis of fluorescent ZnO nanoparticles. Appl. Surf. Sci. 2010, 257, 1756−1761. (8) Baltes, C.; Vukojević, S.; Schüth, F. Correlations between synthesis, precursor, and catalyst structure and activity of a large set of CuO/ZnO/Al2O3 catalysts for methanol synthesis. J. Catal. 2008, 258, 334−344. (9) Behrens, M.; Lolli, G.; Muratova, N.; Kasatkin, I.; Havecker, M.; d'Alnoncourt, R. N.; et al. The effect of Al-doping on ZnO nanoparticles applied as catalyst support. Phys. Chem. Phys. Chem. Chem. Phys. 2013, 15, 1374−1381. (10) Boz, I.; Kaluza, S.; Boroglu, M. Ş.; Muhler, M. Synthesis of high surface area ZnO powder by continuous precipitation. Mater. Res. Bull. 2012, 47, 1185−1190. (11) Chen, G. G.; Luo, G. S.; Xu, J. H.; Wang, J. D. Membrane dispersion precipitation method to prepare nanopartials. Powder Technol. 2004, 139, 180−185. (12) White, C. W.; Aziz, M. J.; Roorda, S.; Moberlychan, W. J.; Haynes, T. E.; Ramaswamy, V. Synthesis of Nearly Monodisperse Embedded Nanoparticles by Separating Nucleation and Growth in Ion Implantation. Nano Lett. 2005, 5, 373−377. (13) Lee, H. B.; Mullings, M. N.; Jiang, X.; Clemens, B. M.; Bent, S. F. Nucleation-Controlled Growth of Nanoparticles by Atomic Layer Deposition. Chem. Mater. 2012, 24 (21), 4051−4059. (14) Chang, C.; Paul, B. K.; Remcho, V. T.; Atre, S. V.; Hutchison, J. E. Synthesis and post-processing of nanomaterials using microreaction technology. J. Nanopart. Res. 2008, 10, 965−980. (15) Sefcik, J.; Soos, M.; Vaccaro, A.; Morbidelli, M. Effects of mixing on aggregation and gelation of nanoparticles. Chem. Eng. Process. 2006, 45, 936−943. (16) Wang, K.; Wang, Y. J.; Chen, G. G.; Luo, G. S.; Wang, J. D. Enhancement of Mixing and Mass Transfer Performance with a Microstructure Minireactor for Controllable Preparation of CaCO3 Nanoparticles. Ind. Eng. Chem. Res. 2007, 46, 6092−6098. (17) Du, L.; Wang, Y.; Luo, G. In situ preparation of hydrophobic CaCO3 nanoparticles in a gas-liquid microdispersion process. Particuology 2013, 11, 421−427. (18) Wang, Y.; Zhang, C.; Bi, S.; Luo, G. Preparation of ZnO nanoparticles using the direct precipitation method in a membrane dispersion micro-structured reactor. Powder Technol. 2010, 202, 130− 136. (19) Huang, C.; Wang, Y.; Luo, G. Preparation of Highly Dispersed and Small-Sized ZnO Nanoparticles in a Membrane Dispersion Microreactor and Their Photocatalytic Degradation. Ind. Eng. Chem. Res. 2013, 52, 5683−5690. (20) Xiulin, L.; Hongyan, X. U.; Lili, Y. U.; Mei, L.; Chengjian, W.; Deliang, C.; Minhua, J. Self-assembly of ZnO nano-particles and preparation of bulk ZnO porous nanosolids. Chin. Sci. Bull. 2005, 50, 612−617. (21) Zhou, Y.; Zhang, M.; Yu, J.; Zhu, X.; Peng, J. Experimental investigation and model improvement on the atomization performance of single-hole Y-jet nozzle with high liquid flow rate. Powder Technol. 2010, 199, 248−255. (22) Ferreira, G.; Garcia, J. A.; Barreras, F.; Lozano, A.; Lincheta, E. Design optimization of twin-fluid atomizers with an internal mixing chamber for heavy fuel oils. Fuel Process. Technol. 2009, 90, 270−278. (23) Wang, Y.; Zhang, X.; Wang, A.; Li, X.; Wang, G.; Zhao, L. Synthesis of ZnO nanoparticles from microemulsions in a flow type microreactor. Chem. Eng. J. 2014, 235, 191−197. (24) Kim, D. S.; Kwak, S. The hydrothermal synthesis of mesoporous TiO2 with high crystallinity, thermal stability, large surface area, and enhanced photocatalytic activity. Appl. Catal., A 2007, 323, 110−118. (25) Sanna, V.; Pala, N.; Alzari, V.; Nuvoli, D.; Carcelli, M. ZnO nanoparticles with high degradation efficiency of organic dyes under sunlight irradiation. Mater. Lett. 2016, 162, 257−260.

4. CONCLUSIONS In this experiment, a new method to synthesize ZnO nanoparticles is presented. Compared to other methods, the process of synthesizing ZnO nanoparticles by continuous gasbased impinging streams has better mixing performance, and it effectively reduces the degree of agglomeration among the particles. Therefore, it can create zinc oxide nanoparticles with the desired properties of high specific surface area and porosity. The operation conditions have a great influence on the quality of the obtained nanoparticles, probably because of their effects on mixing performance and particle aggregation. In addition, the precipitation reaction occurred in unconfined space, avoiding the problem of easy blockage. The throughput of the preparation method is 4.56 g/min. In industry, it is possible to use the direct precipitation method for synthesizing ZnO nanoparticles with better properties. The obtained ZnO nanoparticles with high surface areas indicated excellent photocatalytic activity of methyl orange with 98.2% degradation rate. In the future, we will introduce competitive reactions and other characterization methods to quantify the effect of pressure and gas liquid ratio on the mixing performance. We will also do more experiments to study the influence of impinging streams on mixing intensity for better understanding the process.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03348. The XRD patterns of prepared ZnO and photoluminescence spectrum to obtain optical properties (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Y. Wang. E-mail: [email protected]. *L. Du. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support by the National Basic Research Foundation of China (Grant No. 2013CB733600) and the National Natural Science Foundation (Grant Nos. 21276140, 20976096 and 21322604).

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REFERENCES

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DOI: 10.1021/acs.iecr.6b03348 Ind. Eng. Chem. Res. 2016, 55, 11943−11949